In cellular communications systems there are typically regions where the coverage is difficult or incomplete, for example, within metal-framed structures, or underground. Methods for improving the coverage in regions such as these are known in the art.
In one system known in the art, a repeater is used between a base station transceiver subsystem (BTS) which is able to receive signals in a closed environment, such as a tunnel closed off to transmissions from the BTS. The system down-converts a high radio-frequency (RF) signal from the BTS to an intermediate frequency (IF) signal. The IF signal is then radiated by a cable and an antenna in the closed environment to a receiver that is also located in the closed environment. The receiver up-converts the IF signal to the original RF frequency. Such systems may be used with vehicles moving through a tunnel to enable passengers in the vehicle who would otherwise be cut off from the BTS to receive signals.
In accordance with another system known in the art, a plurality of repeater systems are used between a plurality of BTSs. The systems are used in association with an environment that is closed off to transmissions from the BTSs. Each repeater system down-converts an RF signal from a respective BTS to an IF signal. The IF signal is then transferred by a cable in the closed environment to one or more respective receivers in the closed environment. Each receiver up-converts the IF signal to the original RF frequency. Such systems are used with vehicles moving between overlapping regions in a tunnel, each region covered by one of the BTSs via its repeater system. Thus, passengers in the vehicle who would otherwise be cut off from one or more of the BTSs are able to receive signals from at least one of the BTSs throughout the tunnel.
In yet another system known in the art, a distributed antenna array is used within a region where reception is difficult. The performance of the antenna array is enhanced by generating signal diversity within the array. Each antenna in the array has a differential time delay applied to signals that it receives, thus generating received signal diversity. The differentially-delayed signals are preferably down-converted to an intermediate frequency and are then transferred out of the region via a cable.
Another system known in the art uses a wireless repeater comprising first and second spatially-separated antennas. Both antennas receive a signal from a transmitter, and the signal received by the second antenna has a time delay added to the original signal. The two signals are summed to form one aggregate signal, which is transmitted from a third antenna. A receiver of the aggregate signal is able to reconstruct the signals received by the first and second antennas.
Notwithstanding the above systems for providing coverage in areas where reception is difficult, problems of coverage in such areas continue to exist. The problems are exacerbated by the fact that even when signals in the areas can be detected, the signal quality is in many cases marginal. In other cases, signal levels of a transmission in an area of difficult coverage may vary strongly from moment to moment, so that while at initiation of the transmission the level may be more than adequate, during the course of transmission the level may become less than adequate. There is thus a need for an improved method for detection of signals in difficult reception areas. There is also a continuing need, as demands on cellular networks increase, to increase the capacity of a network without significant increase in the bandwidth requirements of the network.